(1) Field of the Invention
The invention relates to a variable-speed chain drive.
(2) Brief Description of the Prior Art
Variable-speed drives employing belts or chains as means of traction find application in various machines. One known type of infinitely variable-speed drive is the so-called PVI drive in which a disk-link chain runs between two pairs of radially toothed and beveled disks which define drive and driven sprocket wheels. By moving the beveled disks of one beveled disk pair apart and simultaneously moving the beveled disks of the other beveled disk pair together, the path followed by the chain, and thereby the rotational speed of an output shaft connected to one disk pair, can be varied. In a PIV drive, the transversely flexible chain links engage the tooth spaces of the beveled disks and positively transmit the rotational moment introduced at the drive wheel.
PIV drives are complicated in construction and therefore expensive. They also require large physical dimensions. To displace the beveled disks, which can be carried out only during operation, high axial forces are necessary, as well as a relatively long period for adjustment.
Chain drives with a limited number of transmission ratios are known from bicycle design. Their operation is unsatisfactory for various reasons. Transferring the chain from one chainwheel or sprocket to another requires time and is possible only in the absence of load. Speed is lost, particularly in downshifting on hills. Misalignment of chainwheels and sprockets produces chain skew, resulting in substantial material stress. As the chain runs onto a chainwheel or sprocket, lateral friction occurs between teeth and link plates and results not only in fretting, but also in a reduction in efficiency. Chain pitch during shifting does not conform to that of the new chainwheel, so danger exists that the chain will come off. In addition, the speed gradations are not as great as suggested by multiplication of the number of chainwheel and sprocket teeth; diametrically opposite chainwheels and sprockets normally cannot be used together because of their diagonal pull.
The object of the present invention is therefore to create a chain drive with infinitely variable transmission ratio, over a wide range of adjustment and with optimal efficiency, in which readjustment of the transmission ratio can be carried out at standstill as well as under load. Moreover, the chain drive of the present invention is considerably less expensive to produce and physically more compact than prior devices of similar character. Finally, the chain drive of the present invention permits quick and easy adjustment of the transmission ratio.